A truck wheel bearing unit that includes two inner rings and an outer ring is known. Such a wheel bearing unit is delivered to customers with a loosely attached O-ring for sealing the wheel bearing unit to a structure on which it is mounted. When assembled, the O-ring is disposed in a recess in an axial end face of the axially inner inner-ring at a radially inner region such that, in a fully assembled state, the O-ring abuts both on the axially inner-lying inner ring and on a shoulder of a journal on which the bearing unit is applied. In this manner, the O-ring seals in a manner that substantially prevents the penetration of dirt and water onto the bearing seat.